The present invention relates to new laboratory-selected colonies of western corn rootworm (WCR, Diabrotica virgifera virgifera, LeConte) with increased tolerance to maize containing event DAS-59122-7, as well as methods of using such tolerant organisms and the information gathered from such organisms.
Maize (zea mays) is often referred to as corn in the United States. One major problem faced by growers of maize in the United States is the effects of pests, such as WCR, on the yield and standability of a particular maize crop. In an effort to combat pest infestations, various methods have been employed in order to reduce or eliminate pests in a particular plot. These efforts include rotating corn with other crops that are not a host for a particular pest, applying pesticides to the above-ground portion of the crop, applying pesticides to the soil in and around the root systems of the affected crop, and utilizing maize plants incorporating transgenic genes which cause the maize plant to produce insecticidal proteins providing protection from the target pest(s).
A recurring problem with these types of pest management strategies is the development of resistance in the pest population. If a particular pest management strategy is used for a long enough period of time, eventually pests that are resistant to the particular pesticidal strategy utilized will be selected for, and the pest population will eventually be predominantly comprised of pests with resistance to the pest management strategy. Once this occurs, the strategy or particular insecticidal tactic previously used is no longer effective, and efforts must be undertaken to determine a new method to reduce or eliminate the target pest.
Although efforts have been made to slow the development of resistance to pesticides, the evolution of resistance is generally considered inevitable when the pest management tactic applies some selective pressure. Once widespread resistance develops, the chemical (or chemical-class) that resistance has developed against is typically abandoned. The subsequent focus in the research and industrial community is to identify novel pesticides and antibiotics with different modes of action, where positive cross-resistance to previously used toxicants does not occur.
The most commonly-used strategy to manage or slow the development of resistant pests in crops is by use of a refuge. A refuge is a source for susceptible pests to survive on untreated or non-pest resistant sources. In this regard, the refuge permits susceptible pests to survive and grow to adulthood, allowing them to mate with pests exhibiting tolerance or resistance to the pest management strategy, thereby diluting the prevalence of the gene(s) conferring resistance or tolerance. Under currently-accepted guidelines, a minimum of 20% corn refuge is typically used in order that sufficient susceptible pests survive to adulthood. Of course, this method has the obvious drawback of leaving 20% of a corn crop susceptible to pest attack, thereby reducing yield substantially in those plants.
Another alternative to discarding old compounds and continually seeking new compounds is the development of negative cross-resistance strategies to control organisms containing the resistance allele. Negative cross-resistance (NCR) as a strategy for insecticide resistance management refers to a scenario where organisms tolerant to one compound are highly sensitive to another compound and vice versa. For example, if one treats an insect population with a toxin such as pesticide “A,” the number of insects carrying alleles resistant to pesticide “A” will increase in frequency. After numerous generations, insects carrying the “A” resistance allele will comprise the majority of the population. At this time a second toxin that preferentially kills those insects tolerant to the first toxin is used to treat the insect population. Use of the second toxin changes the frequency of the alleles such that the first toxin can again be used to control the insects' population for one to several generations. By alternately deploying the two toxins, a NCR strategy can be used to maintain effective control of the pest while ‘managing’ the resistance alleles in the insect population.
In order to develop negative cross resistance strategies, however, it is necessary to be able to comparatively test susceptible and tolerant organisms to determine compounds which have a higher efficacy against resistant or tolerant organisms as compared to susceptible organisms. This problem is complicated by the manner by which resistant or tolerant pests are typically obtained, namely by direct and potentially unrealistic exposure to the toxin of interest. This type of exposure does not accurately reflect the conditions under which pests will encounter toxins in the field, however, such as in the case of toxins produced by transgenic pest resistant crops.
In addition, when resistant or tolerant organisms are available, the converse effect may be identified, namely positive cross-resistance. In this instance, organisms that are resistant or tolerant to a given toxin also show increased resistance or tolerance to another, different toxin. When this phenomenon is observed, there is a greater danger of resistance or tolerance development in the target pest, as development of resistance or tolerance to one toxin also increases resistance or tolerance to another toxin.
Resistant and tolerant organisms are also beneficial for genetic study. An understanding of the genetics that confer tolerance or resistance to a toxin can be highly beneficial in many respects, including designing new toxins or new versions of existing toxins, understanding the mechanism of resistance development, assisting in determining how resistance may be delayed from a genetic or other perspective and determining how resistance may be delayed if there are multiple and independent traits in the same target pest that confer different levels of resistance.